Natural Cycles is the first FDA-cleared digital cycle tracking app — using basal body temperature data to map the menstrual cycle. We used it for three complete cycles integrated with Oura Ring 4 to evaluate the algorithm, app experience, and data quality.
Natural Cycles uses basal body temperature (BBT) to identify the biphasic temperature shift that follows ovulation. Progesterone released after ovulation raises body temperature by approximately 0.2–0.5°C, which persists until menstruation. The NC° algorithm detects this shift and classifies each day as red (potentially fertile) or green (not fertile, per the algorithm).
This is a product review covering the technology: what the sensor measures and how the algorithm processes it. For any decisions about contraception or cycle management, consult a healthcare professional.
Natural Cycles integrates directly with Oura Ring 4 — importing nightly skin temperature deviation data automatically. This removes the need for a manual morning thermometer reading. Oura's finger-based temperature sensor provides a cleaner signal than wrist-based sensors, making it the best available wearable input for Natural Cycles' algorithm. The integration works reliably and requires only a one-time account connection.
Natural Cycles shows more red (conservative) days during the first 1–3 cycles while building your individual temperature baseline. After 2–3 cycles, the algorithm has enough data to identify your specific biphasic pattern and green days typically increase. Users who expect immediate results will be disappointed; users who understand the data-building process will find it rewarding.
The app is clean and well-designed. The main screen shows today's status (red or green), a cycle calendar, and temperature graph. With Oura integration, logging takes under 10 seconds — open the app to confirm the automatic sync. Symptom logging, mood tracking, and period logging add context over time.
Natural Cycles classifies each cycle day as either red (potentially fertile — use protection or abstain) or green (not fertile, according to the algorithm). The algorithm works by identifying the temperature shift that occurs after ovulation, then using historical cycle data to estimate when the next fertile window will begin. During the initial learning period of 1–3 cycles, the algorithm shows more red days as a precautionary measure while it calibrates to your individual temperature pattern.
As the algorithm accumulates data — typically 2–3 complete cycles — it becomes more confident in your individual biphasic pattern and green days increase. Users with very regular cycles typically see more green days more quickly; users with irregular cycles may require longer calibration periods. The algorithm errs toward showing red days when uncertain — which is the appropriate conservative direction for a health tracking tool.
Most cycle apps (Clue, Flo, Apple Health cycle tracking) use calendar-based prediction — estimating cycle phases from average period length without temperature data. These apps can track period dates and predict approximate cycle phases but cannot detect the actual timing of ovulation in any given cycle.
Natural Cycles is categorically different: it uses actual temperature data from your current cycle to identify when the temperature shift (and therefore ovulation) occurred. This makes it more accurate for cycle awareness purposes and is the basis for its FDA clearance — a regulatory standard no calendar-based app has achieved. For users who want the most data-driven cycle tracking available, Natural Cycles is in a separate category from calendar-based apps.
Natural Cycles costs $99/year or $13/month. The annual plan includes a basal thermometer. With Oura Ring integration, the thermometer is unnecessary — but the annual subscription is required regardless. Over three years: Natural Cycles alone costs $297. Combined with Oura Ring ($349 + $72/year), the total three-year system cost is approximately $780 — the most complete temperature-based cycle tracking system at any price point.